1. Field of the Invention
The present invention relates to an MVA (Multi-domain Vertical Alignment) mode liquid crystal display device, and particularly to a liquid crystal display device in which a quarter wave plate is placed between a liquid crystal panel and a polarizer.
2. Description of the Prior Art
In general, a liquid crystal display device is composed of a liquid crystal panel, in which a liquid crystal is filled in a space between two substrates, and polarizers respectively placed on each of both sides of the liquid crystal panel. A picture element electrode is formed on one substrate of the liquid crystal panel for each picture element, and a common electrode common to each picture element is formed on the other substrate. When a voltage is applied between the picture element electrode and the common electrode, an alignment direction of liquid crystal molecules is changed according to the voltage, as a result, an amount of light, which passes through the liquid crystal panel and the polarizers provided on both sides thereof, is changed. The applied voltage is controlled for each picture element, thereby it is possible to display various images on the liquid crystal display device.
In a TN (Twisted Nematic) mode liquid crystal display device that has been heretofore widely used, the liquid crystal with a positive dielectric anisotropy is used and liquid crystal molecules are twist-aligned between two substrates. However, in the TN mode liquid crystal display device, there is a drawback in which a viewing angle characteristic is insufficient, gradation and contrast are considerably deteriorated, and in an extreme case, displayed images are reversed when the liquid crystal panel is looked at from an oblique direction.
An MVA (Multi-domain Vertical Alignment) mode liquid crystal display device using a liquid crystal with a negative dielectric anisotropy has been known as a liquid crystal display device having an excellent viewing angle characteristic. In the general MVA mode liquid crystal display device, protrusions (alignment control protrusions), which are formed of a dielectric material and extend in an oblique direction, are formed on the common electrode, and slits (alignment control slits) in parallel with the protrusions are formed on a picture element electrode.
In the MVA mode liquid crystal display device, liquid crystal molecules are oriented in a direction perpendicular to a substrate surface in a state where no voltage is applied, and when a voltage is applied to between the picture element electrode and the common electrode, the liquid crystal molecules are inclined and oriented at an angle according to the voltage. At this time, a plurality of regions (domains) where tilting directions of liquid crystal molecules are different from one another due to the slits formed on the picture element electrode and protrusions are formed in one picture element. Accordingly, a plurality of domains where tilting directions of liquid crystal molecules are different from one another are formed in one picture element, thereby it is possible to suppress leakage light in an oblique direction and to obtain a satisfactory viewing angle characteristic. Moreover, the MVA mode liquid crystal display device has a manufacturing advantage that eliminates a process for rubbing an alignment layer.
However, in the MVA mode liquid crystal display device, there is a drawback in which light transmittance is lower and display becomes darker unless strong backlight is used as compared with the TN mode liquid crystal display device. This is caused when the inclined orientations of the liquid crystal molecules in the edge of the protrusions and slits are shifted from predetermined orientations decided by orientations of absorption axes of polarizer and analyzer (hereinafter referred to as polarizers).
In order to solve the above drawback, Japanese Patent Laid-open Publication No. 2001-318371 proposes that circular polarizers (λ/4 plates) are respectively placed between the liquid crystal panel and a polarizer of a back surface side (backlight side), and between the liquid crystal panel and a polarizer of a front surface side (light outgoing side). These two quarter wave plates are arranged in such a manner that their slow axes are orthogonal to each other and each of the slow axes makes an angle of 45° with an absorption axis of each of adjacent polarizers. As mentioned above, the quarter wave plates are respectively placed between the liquid crystal panel and the polarizer of the back surface side, and between the liquid crystal panel and the polarizer of the front surface side, so that light passing through a liquid crystal layer is converted into a circurlar polarized light. In the case of the circurlar polarized light, the influence of the inclined orientation of the liquid crystal molecules in the edge of the protrusions and slits and the influence of the inclined orientation of the liquid crystal molecules at the picture element end portions are eliminated, thereby it is possible to improve light transmittance and provide a bright liquid crystal display device.
However, in the MVA mode liquid crystal display device using the aforementioned quarter wave plates, the viewing angle characteristic is deteriorated as compared with the MVA liquid crystal display device using no quarter wave plate. For this reason, there has been proposed an MVA mode liquid crystal display device in which an optical compensation layer is placed between a quarter wave plate and a liquid crystal panel to suppress deterioration in the viewing angle characteristic. The optical compensation layer is formed to compensate for a negative retardation which a liquid crystal layer has, and there is used a polymer film in which the relationship of Nx=Ny>Nz is established when refractive indexes in an in-plane direction are Nx and Ny and a refractive index in a thickness direction is Nz.
However, sufficient improvement in the viewing angle characteristic cannot be obtained by placing only the optical compensation layer between the liquid crystal panel and the quarter wave plate, and further improvement in the viewing angle characteristic has been demanded.